Development of different means of boundary layer control, particularly as applied to viscous drag reduction, are essential to future development in the fields of ship design, aerodynamics, heat transfer, flow/structure interactions, and acoustics. For most engineering applications, a turbulent boundary layer in the fluid adjacent to a solid surface is the dominant source of the viscous drag. This boundary layer is characterized by swirling, unsteady, seemingly random motion near the surface. Ultimately though, all of the viscous drag on a surface occurs through the viscous activity immediately adjacent to the surface.
Many approaches have been tried for reducing turbulent viscous drag. For applications in water, chemical additives have been found to be particularly promising over the last half century although the true mechanism by which they reduce drag remains debatable. The major problems with such additives are that they must be carried and replenished and there remain concerns about toxicity of additives.
Similarly, the process of micro-bubble cloud injection has been found to dramatically reduce drag but an incontestable mechanism for such reduction has not been discerned. Moreover, the energy cost of generating or injecting the bubbles may not be recovered from the drag benefits.
It has been found experimentally that passive surface textures (e.g. riblets) can reduce the drag on a surface by five to ten percent. However, riblets as well as other very small scale surface textures (e.g., LEBUs) suffer from the problem of biofouling in practice and, anyway, appear limited to only a few percent improvement benefit at best.
There is also much current interest in arrays of active flow manipulators; jets and suction holes, dynamically protruding bumps, flaps, etc. Arrays of micro-electro-mechanical systems (MEMS) elements have been manufactured with various techniques and have been used to measure surface shear stress distributions in fully turbulent boundary layers. The inventor is not aware of any such active flow control devices for drag reduction that have been successfully tested in a realistic application. For sea water application, electromagnetic forcing with arrays of pulsed actuators have also recently been explored.
Therefore, an improved method of reducing the viscous drag on a submerged surface would provide numerous advantages.